The wings of modern transport aircraft operate in the transonic speed regime. The upper aerodynamic surface has a shock wave that although weak in the 1 g cruise condition (steady level flight) does strengthen with increasing lift (due to, for example, a longitudinal manoeuvre) or increasing speed (for example, a dive). The interaction of the wing upper aerodynamic surface shockwave with the local boundary layer has an impact on the flow separation and flow breakdown which can in turn affect loads and handling qualities. The interaction of a shock wave with a laminar boundary layer can have very different characteristics to that with a turbulent boundary layer. There is a significant performance benefit in designing a wing to have laminar flow on the upper aerodynamic surface. However, for such wings the state of the boundary layer in the region of the shock-boundary layer interaction across all points of the flight envelope and throughout the aircraft service life is unknown. This is because a laminar flow may transition to a turbulent flow due to surface discontinuities (steps) and surface roughness caused by, for example, insect impact residue and aircraft ‘wear and tear’, the degree of which may change between scheduled maintenance checks.
The aircraft designer wants to know with confidence the state of the boundary layer at the shock-boundary layer interaction at one or more flight conditions which define structural design load limits. Uncertainty can often lead to conservatism in structural sizing with a corresponding increase in aircraft weight, which affects fuel burn and therefore operating costs for the aircraft.